What is the endocannabinoid system?

This is the part of the biology that explains why a plant compound has any effect on human beings at all. It is also the part most heavily over-interpreted in marketing, so it is worth separating what is established from what is speculation.

What it is

A signalling system made up of three parts: molecules the body produces, receptors those molecules act on, and enzymes that build and break them down. It was discovered because researchers were trying to work out how THC acts.

The two best-described signalling molecules are anandamide and 2-AG. Unlike most neurotransmitters, they are made on demand rather than stored, and they act backwards across a synapse — the receiving cell releases them to tell the sending cell to ease off.

That backwards, on-demand design is the clue to what the system does. It is a regulator. It does not run any single process; it adjusts the intensity of other processes — pain signalling, appetite, mood, sleep, immune activity, memory.

The two receptors

CB1 receptors are found in high density in the brain and central nervous system. CB2 receptors are found mainly in immune cells and peripheral tissue.

• CB1 — abundant in brain regions handling memory, movement, appetite and pain processing. THC's action here explains both its therapeutic effects and its intoxicating and cognitive ones.
• CB2 — concentrated in immune tissue. Interest here centres on inflammation, and drugs acting selectively at CB2 have been an active research target precisely because they would not cause intoxication.

Notably, CB1 receptors are sparse in the brainstem regions that control breathing. That is the accepted explanation for why cannabinoid overdose does not cause the respiratory depression that makes opioid overdose lethal — a real and important difference, and one of the few places where a favourable comparison is warranted.

Where the claims outrun the evidence

Two ideas circulate widely and neither is established. It is worth recognising them, because they are often used to argue that almost any condition might respond.

The first is clinical endocannabinoid deficiency — the proposal that conditions such as migraine, fibromyalgia and IBS share an underlying shortfall in endocannabinoid signalling. It is a legitimate research hypothesis that has been discussed for over two decades. It is not a diagnosis, there is no test for it, and no treatment is licensed on the basis of it.

The second is the argument from ubiquity: because the system touches many functions, cannabis must help many conditions. That does not follow. Paracetamol acts on widely distributed systems too, and is useful for a narrow range of things. Breadth of receptor distribution predicts breadth of side effects at least as reliably as it predicts benefit.

What the evidence supports condition by condition is set out in Can medical cannabis treat my condition?

Patient guide · Written by the Flora House editorial team · Last reviewed: 8 September 2026